Depression in Zulu: tonal effects of segmental features

Zulu, a Bantu/Nguni tone language spoken in South Africa, exhibits a
curious interaction between tone and consonants, in which a specific set
of onset consonants is correlated with extreme lowering of an immediately
following high (H) or low (L) tone. This extreme lowering often causes
realignment of tones and has been called ‘tonal depression’. The
triggering consonants are traditionally referred to as ‘depressors’.

While the phenomenon of tonal depression has been found and studied in
several Bantu languages, neither the set of depressor consonants nor the
mechanism of depression have been satisfactorily characterized. Early
attempts to define depressors as a natural class based on a feature like
[+ breathy voice] are contradicted by the facts (cf. TKF 1987, GPT 1988).
The hypothesis that tonal depression is caused by a L tone associated with
an obstruent (Laughren 1981) is inconsistent with standard assumptions
about the sonority of tone bearing units (TBUs). Proposals for a tone
insertion rule that places an extra L tone on the ‘regular’ tonal tier
cannot explain why tonal depression consistently occurs very late in the
derivation: H tone distribution initially follows its regular patterns
regardless of the presence of depressor consonants, while depressor
induced H tone shift is blocked by depressor consonants.

In this paper, I will argue that tonal depression in Zulu warrants the
introduction of a new feature pair into the feature geometry. This
enhanced feature geometry is not only phonetically motivated, but also
provides a straightforward explanation of otherwise puzzling
characteristics of tonal depression and its effects in the phonology.

I am deeply indebted to Marlys Macken for her detailed
comments on the many drafts of this paper. Anthony Traill kindly supplied
me with literature and gave me invaluable pointers early in the thought
process. I thank Tom Purnell for comments on earlier versions and frequent
discussions. All errors are, of course, my own.